Studies on the ecological life cycle of the native winter annual grass Alopecurus carolinianus, with particular reference to seed germination biologyin a floodplain habitat

Alopecurus carolinianus is a widely-distributed native winter annual grass that grows in the same cultivated floodplain habitat as the narrowly-endemi c winter annual Lesquerella lescurii (Brassicaceae). Most seeds of A. carol inianus buried under nonflooded conditions and exposed to seasonal temperat ure changes had an annual dormancy/nondormancy cycle, being dormant in spri ng and nondormant in autumn. However, 1-16% of the seeds exhumed in spring could germinate at 15/ 6 and 20/10 degreesC. Nondormant seeds were not indu ced into dormancy by a 12-wk period of burial at 5 degreesC; mean monthly m inimum temperatures were about 1-3 degreesC when buried seeds entered dorma ncy. Seeds flooded in October, November, or December under natural temperat ure conditions germinated to 61-99% at 15/6, 20/10, and 25/15 degreesC the following May, but those flooded in February, March, or April germinated to only 1-23% at these temperatures. Thus, flooding prior to the time that te mperatures are low enough to induce dormancy could result in many seeds bei ng capable of germinating in spring. Plants flowered under short and long d ays and had little or no vernalization requirement for flowering. Consequen tly, plants from spring-germinating seeds can complete their Life cycle if not destroyed by soil cultivation. The species has a small, relatively shor t-lived persistent seed bank. In contrast, buried seeds of L,. lescurii hav e an annual dormancy/nondormancy cycle with dormancy induced at temperature s of 20/10 and 15/6 degreesC in October and November (Baskin t al. 1992). L esquerella lescurii also has a large, long-lived seed bank, which ensures p ersistence at the site if no seed production occurs in spring. Although flo oding may destroy all A. carolinianus plants from autumn-germinating seeds, it also prevents many seeds from entering dormancy. Production of seeds by A. carolinianus plants from spring-germinating seeds would help ensure per sistence at the site and compensate for lack of prolonged seed viability.